The present invention is directed to an integrated process for producing isocyanates from phosgene and at least one amine in which chlorine generated by electrochemical oxidation of the hydrogen chloride produced in the course of the phosgenation process is recycled to produce phosgene.
Chlorine is very commonly used as an oxidizing agent in the production chain in the preparation of many organic compounds and in the preparation of raw materials for the production of polymers. Hydrogen chloride is frequently produced as a by-product. For example, chlorine is used in isocyanate production, hydrogen chloride being formed as a by-product. Additional use can be made of the hydrogen chloride, for example by marketing the aqueous solution (hydrochloric acid) or by using it in syntheses of other chemical products. The full amounts of hydrogen chloride that are produced cannot always be marketed or used for other syntheses, however. Furthermore, hydrogen chloride can only be used for syntheses if it has first been purified by appropriate means. On the other hand, its marketing is generally only cost-effective if the hydrogen chloride or hydrochloric acid does not have to be transported over long distances. One of the most common possible uses for the hydrogen chloride that is formed is its use as a raw material in PVC production, wherein ethylene is oxychlorinated with hydrogen chloride to form ethylene dichloride. Disposal of the hydrogen chloride, e.g. by neutralization with alkaline solution, is unappealing from an economic and ecological perspective.
A recycling process for the hydrogen chloride and the return of the chlorine and/or hydrogen to the production process in which the hydrogen chloride is produced is therefore the desired mode of operation. Recycling processes include the catalytic oxidation of hydrogen chloride, by the Deacon process for example, the electrolysis of gaseous hydrogen chloride and the electrolysis of an aqueous solution of hydrogen chloride (hydrochloric acid). Thus an integrated process for producing isocyanates and catalytic oxidation of hydrogen chloride by the Deacon process is disclosed in WO 04/14845, for example, and an integrated process for producing isocyanates and gas phase electrolysis of hydrogen chloride is disclosed in WO 97/24320.
A review of electrochemical recycling processes is given in the article “Chlorine Regeneration from Anhydrous Hydrogen Chloride” by Dennie Turin Mah, published in “12th International Forum Electrolysis in Chemical Industry—Clean and Efficient Processing Electrochemical Technology for Synthesis, Separation, Recycle and Environmental Improvement, Oct. 11-15, 1998, Sheraton Sand Key, Clearwater Beach, Fla”.
Catalytic hydrogen chloride oxidation by the Deacon process as a recycling method, as described in WO 04/014845 for example, has a number of processing disadvantages. For instance, the heterogeneously catalyzed hydrogen chloride oxidation can only be adjusted to different load states within certain limits. The Deacon process is markedly more sensitive to load changes than electrolysis. Changing the capacity of an industrial plant for catalytic hydrogen chloride oxidation is also complicated.
A further disadvantage of catalytic hydrogen chloride oxidation is that the catalyst used for the reaction is exceptionally sensitive to impurities in the hydrogen chloride. The recycling capacity falls dramatically due to a loss of activity of the catalyst. At the same time, the lower conversion of hydrogen chloride oxidation in the reactor makes it more difficult to recover the reaction gases emerging from the reactor (oxygen, hydrogen chloride, chlorine, water). Taken as a whole, this reduces the cost-effectiveness of the catalytic oxidation process significantly.
A process is described in WO 97/24320 and EP 876 335 A in which the hydrogen chloride formed during isocyanate production is converted to chlorine by gas phase electrolysis and the chlorine is returned to phosgene production for preparation of the isocyanate. In the special case of the preparation of toluene diisocyanate TDI), hydrogen is also returned to the production of toluene diamine (TDA). The conversion of hydrogen chloride into chlorine by electrolysis in the gas phase has not yet been tried on an industrial scale and has the disadvantage that industrial performance places increased technical demands on the plant components, in terms of their resistance to pressure for example, and is also associated with increased safety costs. A further disadvantage is that if the hydrogen chloride is not completely converted, a further process step has to be performed in which the chlorine that is formed is separated from excess hydrogen chloride. According to EP 1 106 714 A, oxygen is added to the gaseous hydrogen chloride to improve conversion in gas phase electrolysis. The disadvantage here is that with incomplete oxygen conversion, the chlorine that is formed must be freed from hydrogen chloride and additionally from oxygen, by, e.g., total liquefaction.
Furthermore, according to WO 97/24320 and others, so-called solid electrolyte systems, e.g. Nafion® membranes in which the anode and cathode are positioned on either side of the ion-exchange membrane can be used. The anode and cathode can be gas diffusion electrodes, for example. Alternatively, the catalytically active material acting as the anode or cathode can be incorporated into the ion-exchange membrane or applied to the ion-exchange membrane. The disadvantage here is that if the ion-exchange membrane or the catalytically active material is contaminated or damaged, the entire unit, comprising the ion-exchange membrane and the catalytically active material of the electrodes, must be replaced.
The electrochemical oxidation of an aqueous solution of hydrogen chloride using a gas diffusion electrode as the cathode is described for example in WO 00/73538 and WO 02/18675. In these disclosed processes, rhodium sulfide is used as the catalyst for oxygen reduction at the cathode. According to WO 02/18675, this catalyst is largely resistant to organic constituents which can be present in the hydrochloric acid as impurities and which derive from upstream synthesis steps, for example. The organic constituents travel from the anode chamber to the cathode chamber via the ion-exchange membrane. Over an extended electrolysis running time, organic compounds lead to a rise in voltage, which has a negative impact on the cost-effectiveness of the process. In order to remove organic constituents, purification of the hydrochloric acid using activated carbon and optionally additionally using an ion-exchange resin, e.g. a molecular sieve, is proposed in WO 02/18675.